Birth and early evolution of a Jurassic escarpment: Monte Kumeta Western Sicily

Abstract

The accurate reconstruction of the facies architecture in the Jurassic succession of Monte Kumeta, coupled with a detailed biostratigraphy, allow to define dynamics and genetic factors controlling the conversion of a Bahamian-type carbonate platform to a pelagic escarpment. A change from tidalites to oolites i.e. from the restricted, interior lagoon to a more open-marine sandy depositional environment, records the establishment of a basin south of the Monte Kumeta sector in late Hettangian-Sinemurian times. The oolitic limestones are overlain by earliest Carixian bioclastic grainstones and packstones with micritized grains and by wackestones with radiolarians and sponge spicules, organized in thin sand prisms. The decrease of carbonate productivity indicated by these sediments records the dissection of the platform and the subsequent isolation of a submarine topographic high in the Monte Kumeta sector. Though based only on indirect evidence, it is suggested that a tectonically controlled scarp must have existed between the Monte Kumeta “high” and the basin. Progressive northward retreat of this scarp resulted in the conversion of a shallow platform sector into a gradually steepening slope, along which the distribution of sediments was controlled by repeated tectonic and gravity-induced modifications of the topography of the substrate. Vertical and lateral changes and geometrical relationships of the recognized lithofacies suggest that they were deposited on a stepped surface brought about mainly by, repeatedly reactivated basin ward dipping normal faults. This scenario is clearly reflected by the relationship of platform strata and the overlying encrinites of Carixian/Domerian age. The encrinite bodies show again a prismatic geometry, becoming thicker towards the south and filling the first generation of neptunian dykes. The top of the encrinites is marked by a peculiar jagged dissolution surface with dm-scale pinnacles capped by a thick ferromanganese crust. The formation of this peculiar surface could have been controlled by complex changes in water chemistry probably related to the Early Toarcian anoxic event. The crust itself is dissected by faults of decimetres to metres of throw, sometimes organized into small-scale positive flower structures. In the hollows/depressions of this highly articulated substrate pelagic sediments of Bajocian to Oxfordian age were deposited. They display a clearly onlapping relationship to the encrinites and to the carbonate platform beds. Their thickness rarely exceeds 4 to 5 meters and they are present also as neptunian dykes filling a dense network of fissures. During Late Callovian and Oxfordian times synsedimentary tectonics has intensified resulting in an increase of the inclination of the slope. This led to more and more abundant, gravitationally controlled deformations (slumping and sliding) of semi-lithified and unlithified sediments along the Monte Kumeta escarpment.